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SODIS Batch System

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UV Disinfection
Sunlight-based treatment at scale
Current
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SODIS Batch System
Organized bottle-based treatment

A SODIS batch system treats 100-200 liters a day by designating 50-100 clear PET bottles, filling them every morning from clear water, laying them flat on an angled reflective surface for 6 hours of sun, collecting them in the evening, and using them in date order. Because each bottle is out of service while it is exposed, a steady output of 150 liters a day takes about 150 bottles of 2 liters in all, and overcast days need 225.

Part of Water Treatment at Scale

A SODIS batch system is an organised daily routine of filling, exposing, collecting and using a fixed set of labelled clear plastic bottles to produce a steady supply of sunlight-disinfected drinking water.

A Batch System Needs More Bottles Than The Daily Output Suggests

The carrier’s recipe is simple: designate 50-100 clear PET bottles for SODIS, build an angled reflective surface such as a corrugated metal roof facing the sun, fill all bottles each morning, lay them out, collect each evening, label bottles with the date and use them in order. The output is 100-200 liters a day in sunny climates. The details of treating a single bottle are in SODIS Method.

The carrier’s 50-100 bottles are the ones being exposed. Others are in use, waiting to be used, or being refilled. The count that matters is the whole stock.

What You Need

  • Clear PET bottles (recycling symbol 1) of up to 2 liters, that held food or drink only, not scratched or cloudy
  • Clear water, settled and filtered, passing the read-through-the-bottle test
  • An angled reflective surface, facing the sun, free of shade
  • Permanent marker or tags for dates
  • A log sheet

Method: The Daily Cycle

Step 1: Morning. Fill each bottle three-quarters full, cap and shake for 20-30 seconds, fill to the top and cap. Check that the text read-through test is passed.

Step 2: Lay the bottles flat on the surface, at least 5 cm apart so none shades another. Write the date and the start time on each bottle.

Step 3: Exposure. 6 hours in full or partly cloudy sun; 2 consecutive days if overcast; none in heavy rain or dense cloud. Log the weather.

Step 4: Evening. Collect the bottles that have finished, move them to the store, keep them capped.

Step 5: Use in date order. Drink from the bottle or pour into a clean cup. Bottles not yet fully exposed stay out until they are.

Step 6: Retire scratched or clouded bottles at the weekly check.

Worked Example: Stocking A 150 Liter Rack

Inputs: demand for treated drinking water 150 L/day; 2 L bottles; bottle laid flat about 30 cm × 10 cm, plus the 5 cm gap (assumed bottle dimensions).

  • Bottles used per day: 150 ÷ 2 = 75.
  • Sunny pattern. 75 bottles exposing (today’s batch) and 75 in the store or in use (yesterday’s batch): 150 bottles.
  • Overcast pattern. Each batch needs 2 days, so two batches are exposing at once, 150 bottles, with 75 in use: 225 bottles.
  • Surface area, one batch of 75: each bottle takes about (0.30 + 0.05) × (0.10 + 0.05) = 0.35 × 0.15 = 0.0525 m². Then 75 × 0.0525 = 3.9 m². For two batches, 150 × 0.0525 = 7.9 m².

So the carrier’s roof is at least a few square meters of sun-facing metal, bigger in cloud, with a spare stock of bottles. Bottles that last in daily use need replacing: scratches and cloudiness reduce UV transmission.

Cloudy Days Decide The Design

The system’s output halves in overcast weather because each bottle takes 2 days. A sustained cloudy week with nothing in store runs the community out. The reserve is a day’s stock at least, a second treatment (boiling or chlorine) for runs of dull weather, and a rule that decides who drinks first (Dry Season Backup).

Common Mistakes

  • Counting only the exposed bottles. There must be a stock behind them.
  • Filling with cloudy water. The light never reaches the pathogens.
  • Shading from a neighbouring roof or tree during the afternoon.
  • Mixing exposed and unexposed bottles in one crate. Keep separate, date everything.
  • Reusing bottles that held chemicals or detergent.

Safety

A wrong bottle or a skipped hour can make water look treated when it is not

Water treated by sunlight looks identical to untreated water. Use dated labels, a log, and a strict rule that only bottles with the full exposure go to the store. Do not use bottles that once held fuel, pesticide or detergent. Heated bottles can scald and metal roofs can burn: use gloves or cloths, and take care on roofs.

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